Gila Monster (Heloderma suspectum)

Main sources: Stebbins, Robert, 1985, A Field Guide to Western Reptiles and Amphibians, Houghton Mifflin; Behler, John & Wayne King, 1979, The Audubon Society Field Guide to North American Reptiles and Amphibians, NY: Alfred Knopf; Beck, Daniel, 2004, "Venomous Lizards of the Desert", Natural History 113:6:32-7

We sometimes see Gila Monsters in Saguaro-Juniper lands. These are the only known venomous lizards, and include only two species (the other is the Mexican Beaded Lizard, a closely similar animal found further south in Mexico). Gila Monsters have large and heavy bodies, massive triangular heads, and short limbs with strong curved 5-fingered claws -- see this image close-up below that displays them.

The colors of their bodies are mottled and blotched, with black predominating on the back, alternating with pink, orange, or yellow. They are mainly but not exclusively nocturnal, and like being near water, but they are often seen on ridge-tops as well. They eat eggs, small birds, mammals, and lizards, and insects. They are not aggressive unless molested, and their venom is rarely fatal to humans (but anyway, hands off! -- the pain from their bite is said to be extreme). Surprisingly, they are more closely related to snakes than they are to most other lizards (though their venom systems have evolved independently, being seated in the Monster's lower jaw and not relevant to subduing prey, rather serving defensive and digestive functions). Also surprisingly, the Monitor Lizards of the Old World -- consisting of more than 30 species distributed from Africa through Asia all the way to Australia, including the 9-foot-long Komodo dragon which kills and eats humans and other large mammals -- are their closest living relatives.

Recent intensive studies show that, while Gila Monsters can merely produce a fast shuffle while walking and their metabolic rates are extremely low (among the lowest ever measured in lizards), they forage quite widely during their occasional bursts of activity, can walk at half a mile per hour for hours, and when competing for mates the males "can sustain prolonged bouts of ritualized combat" (Beck p. 35), sometimes wrestling continuously for more than 15 hours. This high aerobic capacity -- higher than most other lizards -- is made possible by their highly conservative overall energy expenditures -- 90% of their time is spent in their burrows, between moments of supersized eating when one may eat 1/3 of its body weight at one sitting -- enough to last it 1/3 of a year.

Gila Monsters select shelters where reduced temperatures lower their metabolic rates (thus conserving energy), and when they go into dry season dormancy they also prefer shelters that are relatively humid because they lose water faster across their skin than do other lizards.

In our area, the two shown below are fairly typical, the light colors varying from quite pink to almost yellow. The one on the left was seen at the Cow Camp in July 2001, the one on the right crossing Cascabel Road near Pool Wash in May 2002. Note its very fat tail, indicating it has eaten well not too long ago -- this is where much of its food energy is stored. This one on the right was typically ready to fight: when the vehicle approached, a large raven was accosting it in the middle of the road, trying to peck at it, and its head was raised and snapping back at the head of the bird. The raven flew off when the car stopped, leaving its prey-rival to lumber on to the side of the road. It showed no signs of injury. Note how both of these animals face the photographer -- this is an invariable posture when the Gila Monster regards itself as cornered. Click on left image to enlarge; on right image for an alternative closeup.


Here below are two heads -- on the left, one seen marching along the edge of Cascabel Road near the Johnny Lyon Hills Narrows in May of 2000; on the right, one who lumbered up to a group of workers resting by streamside in Upper Hot Springs Canyon in May of 1996:


This one below was seen on a lower Hot Springs Canyon high terrace in September of 2001, and to judge by the slenderness of the tail, must have been hungry: it was also moving fast.

Stebbins distinguishes two subspecies: the reticulate and the banded. The reticulate has patterns of mottled coloring in which crossbands are nearly obscured; there are dark tail bands, but these are mottled, and the light interspaces of the tail show mottling also. The banded in contrast show a body with four black saddles or irrregular double crossbands on the body, and a tail with 5 dark bands, little or not at all mottled, and mottling slight or absent in the interspaces. (This latter pattern is apparently the form appearing in juveniles of both subspecies.) By these standards, all of the above Gila Monsters are probably reticulates, which fits with Stebbins' map showing the banded to occur in a more limited distribution in the west and northern part of the state, the reticulate more widely distributed in central and southern Arizona and south into Sonora.

However, this one -- seen crossing the Cascabel Road in late April 2006 -- does look like the banded subspecies, but it was also small, probably a juvenile. Unfortunately, as the animal was waddling at top speed seeking the roadside embankment, the photographer's haste to capture an image cut off the end of the tail in the initial shot:

As it clambered up the embankment, its head obscured by brush, the emboldened photographer briefly pulled its tail (whose tip was black) in order to slow its departure. This stimulated a loud explosive hissing, and the lizard turned to face the offender, resulting in the following images. Below left, the five black bands are clearly visible behind the rear legs. Below right, you can see that the very tip of the tail is indeed black. (Click on each image to enlarge it.)


Any photographer who pursues these animals is likely to become fascinated by their exquisite beauty. For example, observe these close-ups. Below left, a hind leg. Note the slender grace of the paw and the fine finishing of each claw. Below middle, the banding of the tail shows remarkable precision. Below right, the torso, reflecting probable hunger, is decorated with a complex calligraphy. Click on each image to enlarge it.


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